1. Field of the Invention
The present invention relates to a high resolution displacement detector utilizing a diffraction grating, and is particularly concerned with a device for detecting a dislocation of the diffraction grating formed on wafer, mask and the like, or an alignment error arising between the mask and the wafer.
2. Related Background Art
As disclosed in U.S. Pat. No. 4,828,392, this kind of displacement detector includes a system known hitherto such that a diffraction grating is formed on a substrate such as wafer or the like, interference fringes are formed in line in a pitch direction of the diffraction grating by applying coherent two luminous fluxes thereto from the direction different each other, and a displacement and dislocation of the diffraction grating are detected with reference to the static interference fringes. Further, the aforementioned patent discloses that when edges of each element of the diffraction grating on the substrate are symmetric, intensities of diffracted rays from the diffraction grating are obtained on various degrees individually beforehand to a data base, and an error due to the asymmetry is corrected according to the data base at the time when the dislocation is detected practically.
Further, a method for detecting a dislocation of the diffraction grating on light heterodyne system by flowing interference fringes irradiated on the diffraction grating at high speed in one direction is also disclosed, for example, in U.S. Pat. No. 4,710,026. According to the art disclosed in the patent, a constant frequency difference is provided to coherent two luminous fluxes, and the two luminous fluxes are crossed on the diffraction grating at a predetermined angle, thereby obtaining the interference fringes flowing in one direction at a velocity according to the frequency difference. Then, when detecting the dislocation, an interference intensity of the diffracted beams of a degree generated in the same direction from the diffraction grating is detected photoelectrically, and thus a measuring photoelectric signal of the frequency equal to a frequency difference of the two luminous fluxes is obtained. On the other hand, a reference signal of the frequency equal to a frequency difference of the two luminous fluxes is prepared separately, a phase difference of the measuring photoelectric signal to the reference signal is detected, thereby measuring a dislocation (displacement) of the diffraction grating in a pitch direction within .+-.1/4 pitches of the grating.
Further, that for which a dislocation detector of such light heterodyne system is incorporated as TTR (through-the-reticle) alignment system in a projection exposure device (stepper) for manufacturing semiconductor elements is disclosed in Japanese Patent Application Laid-open No. 63-283129. With one shot domain on the wafer aligned to a projected image of a reticle pattern at .+-.1 .mu.m or so, the TTR alignment system is normally used such that a grating mark of the shot domain is detected to obtain a dislocation coming 1 .mu.m or below, and a wafer stage or a reticle stage is moved fine so that the dislocation is corrected. Thus, in the TTR alignment system, the two luminous fluxes and the wafer (grating mark) do not move relatively with each other when detecting the grating mark, however, an interference fringe on the grating mark keeps flowing at all times, therefore the phase difference can be measured successively.
In case the wafer is subjected to measurement, the wafer in an exposure process has a resist layer applied normally on overall surface thereof.
Accordingly, the diffraction grating mark on the wafer surface will be irradiated. The resist layer is thin at 1 .mu.m or so, however, a laser beam or the like with fine monochromatism is used for the two luminous fluxes, therefore there may be a case where an intensity of the diffracted beam (.+-.primary light, for example) generated from the grating mark changes to excess due to a slight change in thickness of the resist layer. Further, in the wafer and others subjected to a series of process, a duty (ratio of line width to space width) of the grating mark itself may change from 50%, therefore an intensity of the diffracted beam (.+-.primary light) to detect changes in this case.
A further defect is such that when influences of both the resist layer and grating duty, are an unfavorable appropriate detectable intensity of the diffracted beam (interference ray of .+-.primary lights) is almost not obtainable.